Scanning probe microscopes (SPMs) are a powerful family of microscopes used to form images of nanoscale surfaces and structures, down to atomic dimensions. Images are attained by scanning an extremely sharp probe tip across the surface of a sample, typically back and forth in a raster pattern, via the use of a piezoelectric actuator. The probe's tip can be as sharp as a single atom. It can be moved precisely and accurately across the sample surface, even atom by atom. The probe can be moved, or scanned, in relation to a stationary sample, or the sample can be scanned in relation to a stationary probe. The movement resolution varies somewhat from technique to technique, but some probe techniques reach atomic resolution due to the ability of piezoelectric actuators to precisely execute motions on electronic command.
One common example of an SPM is the atomic force microscope (AFM), which scans the sharp probe tip, attached to a flexible spring lever (commonly called a cantilever) in relation to the sample surface. The probe tip, the cantilever, and the support substrate are together herein referred to as “the probe”. By measuring motion, position or angle of the free end of the cantilever, many properties of the surface may be determined including surface topography, local adhesion, friction, elasticity, the presence of magnetic or electric fields, etc. In many such cantilever-based SPMs the probe is mounted to the end of a piezoelectric crystal or to a piezoelectric driven flexure scanner, both of which induce precise and rapid movements of the probe.
In the case of AFMs and other cantilever-based SPMs, when the probe tip is near the sample surface the cantilever is deflected by a force. Such SPMs can measure deflections caused by many kinds of forces, including mechanical contact, electrostatic forces, magnetic forces, chemical bonding, van der Waals forces, and capillary forces. The distance of the deflection is typically measured by a laser that is reflected off the top of the cantilever and onto a position sensing photodiode. SPMs can detect differences in height that are a fraction of a nanometer, about the diameter of a single atom. During this scanning process, a computer gathers data that are used to generate an image of the surface. In addition to visualizing nanoscale structures, some kinds of SPMs can be used to manipulate individual atoms and move them to create specific patterns.
Other SPMs include the scanning near-field optical microscope, the scanning capacitance microscope, the scanning magnetic force microscope, the scanning electric field microscope, the scanning ion conductance microscope, the scanning tunneling microscope (which typically does not utilize a cantilever), and several others.
The piezoelectric actuators are typically made of naturally occurring crystals such as quartz, berlinite (AlPO4), topaz, tourmaline-group minerals, barium titanate, and lead titanate, each of which is relatively brittle and expensive.
In many of these scanning probe techniques the probes become quickly damaged, contaminated or dull, and must be replaced to regain the ability to make high quality measurements of a sample. Also, frequent interchange of different specialized probes for detecting various sample characteristics may be required. Exchange of the probes happens as frequently as every hour in a busy laboratory. One of the more difficult and time consuming aspects of operating a scanning probe microscope (SPM) entails the manual exchange of these extremely small and difficult to handle probes. The cantilevers on these probes may be as long as the width of a human hair, and extend from a support substrate that may typically be about 3.4 mm long, 1.6 mm wide, and 0.3 mm high.
In the prior art, probe exchange has been done manually by an operator who removes the old probe and reinstalls a new probe. Changing probes demands skill and dexterity and they are easily damaged in the process. Because of the delicate nature of the probes, replacement of the probe in some SPM designs may take many minutes, as described in U.S. Pat. No. 5,376,790, assigned to Park Scientific Instruments. During this time, the SPM instrument is unavailable for use, so minimal probe exchange time is essential for high sample throughput. Additionally as scanning probe microscopes become more widely used, there is increasing pressure to develop instruments that can be operated more quickly and used by less-skilled operators, and even driven automatically without operator intervention. Achieving these goals will open the SPM market to a significantly larger customer base.
Most of the prior art scanning probe microscopes have no provision for automatic probe exchange. These systems require that an old probe be removed by hand and a new probe installed by hand in its place. A few systems have multiple probes mounted on carousels or similar rotatable carriers. Additionally, U.S. Pat. No. 5,705,814 to Young discloses means for automatic probe exchange including a clamp on a probe mount which is opened and closed by external actuators. In this case, the use of external actuators can impart forces upon the piezoelectric crystal or piezoelectric driven flexure scanner, sufficient to cause damage to said piezoelectric crystal or piezoelectric driven flexure scanner.
U.S. Pat. No. 5,705,814, referenced above, also discloses a method for probe exchange utilizing a vacuum to directly attach the probe to a mounting interface. While this is effective in cleanroom environments free from dust and airborne particulates, it is not effective in ambient conditions typical of laboratory environments. In this case airborne particulates become lodged in the interface between the probe and its mating surface. These enable microscopic vacuum leaks within the interface causing turbulence and imparting vibration to the probe, resulting in unacceptable system noise. The need for a clean room environment greatly restricts the usefulness of such systems, since most labs that utilize SPMs are not so equipped.
Another probe exchange system described in U.S. Pat. No. 8,099,793 to Jo discloses a magnetic pick-up which requires the use of an intermediate metal probe carrier acted upon by competing magnetic fields. The requirement of such an intermediate probe carrier necessitates an initial undesirable time-consuming process. That is, aligning and adhering the given probe to the metal probe carrier. U.S. Pat. No. 7,597,717 discloses a rotatable multi-cantilever scanning probe microscopy head and methods of changing of probes via rotation of multi-probe cartridges. Additionally, U.S. Pat. No. 8,689,360 discloses a similar rotatable multi-probe holder. While the use of such multi-probe cartridges or multi-probe holders may be convenient, these cannot effectively be mounted to high speed scanners which actuate in the Z-axis, since the additional mass detrimentally impacts scan speed.
There is thus a need for a probe exchange device which does not require an intermediate probe carrier, is small and lightweight, is operable under ambient conditions, and which may be actuated frequently and without risk of damaging the highly precise piezoelectric displacement mechanisms.